Aerial systems



April 17, 1956 D. E. CRONIN 2,742,640

AERIAL SYSTEMS Filed March 18, 1952 INVENTOR Dew/ ER c (RON/N lTTORNEYHired This invention relates to aerial systems. I

It is well known that on open-ended waveguide may beused for launchingelectro-magnetic' radizitidtiinto space. -If now the waveguide isformed'by a holethrough a metal body so that there is an' electricallyconducting surface extending for an appreciable distance from thewaveguide'aperture in a plane inclined, for example at right angles, tothe waveguide axis, it is found that-currents set -up in the conductingsurface tend to distort the radiation polar diagram from that of thewaveguide alone since there is radiation both from the waveguide andfrom the conducting surface.

=Iti'isone object of the present invention to provide an improved aerialsystem in which this distortion of the radiation polar diagram isreduced.

According to the present invention, an aerial system comprises awaveguide which extends into a metal body and has its open end flushwith the outside surface of the body, there being a gap between the bodyand a metal wall of the waveguide, the end of this gap remote from thesaid surface being bridged electrically and the gap being of such depththat, at the operating frequency of the aerial system, there ispresented a high, preferably infinite, impedance between the outsidesurface of the body and the said end of the waveguide wall.

If the waveguide is of circular cross-section,'it is preferred that thegap shall completely encircle the waveguide.

One arrangement of an aerial system for launching circularly-polarisedelectromagnetic radiation will now be described by way of example withreference to the two figures of the accompanying drawing. Figure 1 showsa plan view of the aerial system while Figure 2 shows a cross-section atthe line II--II in Figure 1.

Referring now to the drawing, the aerial system comprises a waveguide 1which is adapted to launch; the circularly-polarised radiation in thedirection of the arrow 2. The waveguide 1 is of circular cross-sectionand is formed in a brass member 3. The member 3 has an annular flange 4on which is seated a metal, for example brass, plate 5.

The metal of the member 3 surrounding the waveguide wall 6 is cut awayso as to form an annular gap 7 surrounding the waveguide 1. The end 8 ofthe waveguide wall 6 is flush with the outside surface 9 of the plate 5.The sides 11 and 12 of the gap 7 are parallel to the waveguide axis 13,and the sides 11 and 12 provide a co-axial transmission line which isshorbcircuited at its end remote from the surface 9. The depth of thegap 7 is such that, at the frequency at which radiation is to belaunched from the waveguide 1, the effective impedance presented by theopen end of said transmission line is substantially infinite. For thispurpose the depth of the gap 7 has an electrical length approximatelyequal to one quarter wavelength at the operating frequency.

It will. be appreciated, therefore, that there is a substantiallyinfinite impedance between the end 8 of the 2,742,640 Patented Apr. 17,19 56 waveguide wall 6 and the surface 14 of the 'part'15 of the member3 which surface is co-planar with the surface 14 so that the surfaces 9and 14 effectively form a'single conducting surface. This high impedancetends to prevent any appreciable current being set up in the metalsurrounding the waveguide 1, when the aerial system is radiating, whichwould affect the-polar diagram of the systernz I 5 The open end of thewaveguide 1 may be closed by a member which forms a window across thewaveguide.

' Moreover'the waveguide 1' and/or the gap 7 may be filled with soliddielectric'material, for example polythene.

l Although theoretically it is necessary for the gap 7 to have anelectrical length of a quarter wavelength, it is found that, in theaerial system described above, there is noappreciable distortion ofthepolar diagram due to currents in the surface 9 when the operatingfrequency lies anywhere in a-band of up to ten per cent of the optimumfrequency. I

Instead, as in the above arrangement, of providing the waveguide and itssurrounding gap in a unitary member,' the waveguide may alternatively beprovided by a metal tube-which fits into a metal body. In thisarrangement the'open end of the waveguide member'is again flush with theoutside surface of the body and the body itself is' shaped so as toprovide a gap surrounding the waveguide member,-the waveguide member andthe body being'secured together, for example by soldering, to providethe required short-circuit across the end of the gap.

Aerial systems in accordance with the invention may be used for eithertransmitting or receiving electromag netic radiation.

I claim:

1. An aerial system comprising a metal body having an externalelectrically conductive surface facing unobstructed space, a metalwaveguide which extends into the body from said external surface, saidwaveguide having an open end which is flush with' said external surfaceof the body and launches electro-magnetic radiation into space, theshape and size of the cross-sectional configuration of the open end ofthe waveguide being the same as the shape and size of thecross-sectional configuration of the waveguide within the body, saidexternal surface of the body extending for an appreciable distanceradially outwardly away from the open end of the waveguide, there beinga gap between the body and the wall of the waveguide immediatelyadjacent the open end thereof, and means electrically bridging the endof the gap re mote from said external surface at a depth such that atthe operating frequency of the aerial system a high impedance ispresented between said external surface of ,the body and the wall of thewaveguide at the open end thereof.

2. An aerial system comprising a metal body having an externalelectrically conductive surface facing unobstructed space, a metalwaveguide which extends into the body from said external surface, saidwaveguide having an open end which is flush with said external surfaceof the body and launches electro-magnetic radiation into space, theshape and size of the cross-sectional configuration of the open end ofthe waveguide being the same as the shape and size of thecross-sectional configuration of the waveguide within the body, saidexternal surface of the body extending for an appreciable distanceradially outwardly away from the open end of the waveguide, there beinga gap between the body and the wall of the waveguide immediatelyadjacent the open end thereof, and means electrically bridging the endof the gap remote from said external surface at a depth equal toapproximately one quarter wavelength of the operating frequency of theaerial system so as to present a high impedance between saidexternalsurface of the body and the wall of the waveguide at the openend thereof.

3. An aerial system comprising a metal body having an externalelectrically conductive surface facing unobstructed space, a metalwaveguide which extends into the body from said external surface, saidwaveguide having an open end which is flush with said external surfaceof the body and launches electromagnetic radiation into space, the shapeand size of the cross-sectional configuration of the open end of thewaveguide being the same as the-shape and size of the cross-sectionalconfiguration of the waveguide within the body, said external surface ofthe body extending for an appreciable distance radially outwardly awayfrom the open end of the waveguide, there being a gap between the bodyand the wall of the waveguide immediately adjacent the open end thereof,and means electrically bridging the end of the gap remote from saidexternal surface at a depth within ten per cent of one quarterwavelength of the operating frequency of the aerial system so as topresent a high impcdance between said external surface of the body andthe wall of the waveguide at the open end thereof.

4. An aerial system according to claim 3 wherein the waveguide is ofcircular cross section and the gap is a full annulus completelyencircling the waveguide.

5. An aerial system according to claim 4 wherein the waveguide iselongated and its longitudinal axis is perpendicular to the externalelectrically conductive surface of the body.

6. An aerial system as set forth in claim 4 wherein the waveguide andbody are unitary.

7. An aerial system'comprising a metal body having an externalelectrically conductive surface facing unobstructed space, a metalwaveguide which extends into the body from said external surface, saidwaveguide having an open end which is flush with said external surfaceof the body and constitutes the operative end of the aerial system, theshape and size of the cross-sectional configuration of the open end ofthe waveguide being the same as the shape and size of thecross-sectional configuration within the body, said external surface ofthe body extending for an appreciable distance radially outwardly awayfrom the open end of the waveguide, there being a gap between the bodyand the wallof the waveguide immediately adajcent the open end thereof,and means electrically bridging the end of the gap remote from saidexternal surface at a depth such that at the operating frequency of theaerial system a high impedance is presented between said externalsurface of the body and the wall of the waveguide at the open endthereof.

Great Britain Feb. 7,

